# Find the intersection of two functions, $f(x) = \tan x$ and $g(x) = 5x$.

I can't proceed with the normal method to find these ones because they will intersect infinite times. Should I take an interval?

$$f(x)=\tan(x), \qquad g(x)=5x$$

Thank you.

• Do you just have to give one intersection? Can you just say $x=0$ and not bother with the other intersections? – AHusain Oct 16 '18 at 23:46
• You are looking for solutions of the equation $$\tan(x)=5x.$$ First, obviously, $0$ is a solution. Second, both functions are continuous ($f$ is continuous over its domain, that is, for $x\neq n\pi+\frac{\pi}{2}$ for all $n\in\mathbb{Z})$. Define $$h(x)=\tan(x)-5x,$$ so you can use the mean value theorem to approximate a solution. Third, there is several methods for solving the fixed point problem $\tan(x)=5x.$ – Mateus Rocha Oct 16 '18 at 23:58
• What's the "normal method"? It might give some ideas for answers if the question said what you would have done if there were not infinitely many intersections. – David K Oct 17 '18 at 0:35

## 2 Answers

We can easily find the trivial intersection at $$x=0$$ and for $$x>0$$ each one of the others solutions are contained in the intervals

$$n\pi

and can be found by numerical methods.

As gimusi answered, discrading the trivial solution $$x=0$$, you have a root $$x_n$$ such that $$n \pi < x_n < \left(n+\frac 12\right)\pi$$ Plotting the function, you noticed that the solutions are very close to the right bound.

To get rid of the discontinuities, consider instead looking for the zero's of function $$F(x)=\sin(x)-5x \cos(x)$$ You can get quite good estimates of the solution expanding $$F(x)$$ as a truncated Taylor series built at $$x=\left(n+\frac 12\right)\pi$$. Limited to $$O\left(\left(x-\frac{2 n+1}{2} \pi \right)^3\right)$$, this would give as the equation to solve $$9 y^2-5 (2 n+1 )\pi y+2=0\qquad \text{where} \qquad y=\left(n+\frac 12\right)\pi-x_n$$ As a result, $$x_n \approx \frac{2}{9} (2 n+1)\pi+\frac{1}{18} \sqrt{25 (2 n+1 )^2\pi^2-72}$$ Checking for a few values of $$n$$, $$\left( \begin{array}{ccc} n & \text{estimate} & \text{solution} \\ 0 & 1.4325167 & 1.4320322 \\ 1 & 4.6695980 & 4.6695848 \\ 2 & 7.8284421 & 7.8284393 \\ 3 & 10.977358 & 10.977357 \end{array} \right)$$ For $$n >3$$, there is no difference at the level of $$8$$ significant figures.

You could have simpler estimates using the $$[1,1]$$ Padé approximant built at $$x=\left(n+\frac 12\right)\pi$$. This would give as solution $$x_n \approx \left(n+\frac 12\right)\pi-\frac{10 (2 n+1 )\pi}{25 (2 n+1 )^2\pi^2-18}$$ which is as good as the previous one.

If you need more accuracy, use Newton method starting from one of these estimates. The iterates will be given by $$x_{k+1}=x_k-\frac{\sin (x_k)-5 x_k \cos (x_k)}{5 x_k \sin (x_k)-4 \cos (x_k)}$$ For the first root, we should then have $$\left( \begin{array}{cc} k & x_k \\ 0 & 1.4325167001302000019 \\ 1 & 1.4320324138494111588 \\ 2 & 1.4320322362434419819 \\ 3 & 1.4320322362434180905 \end{array} \right)$$ which is the solution for $$20$$ significant figures.

• If you look into more details, you would notice that, for large values of $n$, the distance betwenn the root and the asymptote is almost $\frac 1 {5\pi n}$ – Claude Leibovici Oct 17 '18 at 4:46